1. Field of the Invention
The present invention relates to electrical safety compliance test equipment, and more particularly to an automated line leakage tester capable of performing a plurality of different line leakage and other types of safety compliance tests.
2. Description of Related Art
Line leakage testing measures the amount of current to which a user of an electrical device is exposed during normal operation of the device. In general, the amount of such leakage current depends on the design of the device, and even a non-defective device may have some leakage current. However, if the leakage current is below a threshold, the leakage current is imperceptible and poses no threat.
In contrast, two other types of electrical safety compliance tests measure conditions which are caused by, or which can cause, catastrophic failure of a product, namely ground circuit and insulation integrity. These tests are to be distinguished from line leakage tests in that they generally require application of voltages or currents other than the normal operating voltages and currents for the device be applied. An example of a test instrument capable of carrying out both ground circuit and insulation tests is found in copending U.S. patent application Ser. No. 09/033,958, filed Mar. 3, 1993, herein incorporated by reference.
The present invention concerns line leakage testing, and in particular the problem of meeting different standards for the tests. In addition, the present invention concerns the problem of combining a line leakage tester with a test instrument such as the one described in the above-cited copending application through external links.
The reason for the different line leakage testing standards is that the amount of leakage current that will pass through a person using a device can only be approximated because the effect of the human body on the current, i.e., the impedance of the body, can only be approximated, and different jurisdictions have chosen different models of the electrical characteristics of the body to develop their standards. In order to meet a standard for a particular jurisdiction, or intended use of a product, the circuitry through which the current to be measured is passed must correspond to the model used in the standard. This is not just a matter of setting different acceptable current thresholds, but of using completely different test circuits.
Generally, line leakage testing is carried out by applying normal line power to the device being tested, touching a probe to the device, and measuring the current present in a circuit connected between the probe and a reference. The circuit connected between the probe and the reference is intended to approximate a person touching both the device and a surface corresponding to the reference, and is referred to as a human equivalent circuit. It is these human equivalent circuits that must be varied in order to meet the different line leakage standards.
In addition to requiring different human equivalent circuits, most line leakage test standards include provision for testing leakage under a variety of power line conditions that might occur during use of a product, including open ground and neutral and reversed polarity conditions, and most standard-setting organizations also require products to be subjects to ground and insulation tests in addition to line leakage tests.
A schematic of a circuit recommended by UL for making the measurement is shown in FIG. 1, which is taken from the publication "A Basic Guide To Product Safety Testing" published by Associated Research, Inc. The circuit includes a line input 1 for supplying power to a receptacle 2 into which the device under test (DUT) 3 is plugged upon positioning on an insulating table or platform 4. An AC volt meter 5 monitors the input current while leakage current is measured by a volt meter 6 across a human equivalent circuit 7. A switch Si permits either an open neutral or closed neutral test configuration, while switch S2 reverses the polarity of the power connections to the receptacle, and switch S3 connects the power circuit to the third wire ground prong of the receptacle.
If all persons perceived current in the same way, there would be no need for multiple line leakage current test capabilities in a single instrument, and a single circuit corresponding to circuit 7 shown in FIG. 1 would be adequate. However, the impedance of a person to the leakage current depends on a number of factors, including the size of the person, moisture on the person's skin, and whether the person is standing or lying down. Because these conditions can only be approximated, different agencies have selected different models as best approximating the electrical characteristics of a user of an electrical device.
Examples of different human equivalent circuits are shown in FIGS. 2A-2E. In addition to the illustrated circuits, organizations such as the Underwriter's Laboratories (UL), Canadian Standards Association (CSA), Association of German Engineers (VDE), International Electrotechnical Commission (IEC), British Standards Institution (BSI), and Japanese Standards Association (JIS), and other private and governmental testing agencies have selected a variety of other human equivalent circuits for testing the maximum amount of current that may leak from a non-defective product operating at its normal line voltage. Any product that is to be sold in jurisdictions covered by more than one of these organizations must be tested using multiple different human equivalent circuits.
Furthermore, even within a single jurisdiction, different electrical products may have different line leakage standards to account for the intended use or intended users of the products. For example, products intended for children may require different equivalent circuits then those intended for adults, while products intended for persons in hospital beds may require different human equivalent circuits than similar products used outside a hospital, as evidenced by the differences between FIG. 2D, which is a common standard for non-patient care equipment, and FIG. 2E, which shows the UL standard for patient care equipment.
The present invention is a line leakage test instrument that enables a variety of different products to meet different test standards by including the capability of switching between multiple human equivalent leakage test impedance circuits, under multiple power line conditions, including open-neutral, open-ground, and reversed polarity, and in addition offers multiple probe configurations including probe-to-probe and ground-to-neutral or probe-to-neutral configurations.
Rather than simply adding circuits to the conventional single impedance circuit tester, however, the present invention also addresses various additional problems related to the inclusion of multiple test circuits, including the problem of ease-of-use, safety, cost, and reliability, and furthermore provides external link capabilities so that the same test set-up can be used not only to perform multiple line leakage tests, but also other types of safety compliance tests, including tests for defects in the ground circuit or insulation of an electrical product.
The external link capability greatly increases the versatility of the line tester, but also increases the hazards because each type of test requires different power and test connections. Electrical product ground circuit testing standards generally require currents sufficient to enable measurement of the impedance of the ground circuit (often referred to as "ground bond" testing), while tests of insulation integrity require a voltage at least sufficient to measure the resistance across two electrically isolated points in a device (insulation resistance testing), or to stress the insulation in the device in order to measure the resulting leakage current (dielectric withstand or hipot testing). In order to minimize these risks, it is preferable to enable centralized control of all tests, which in the case of separate test instruments is best carried out by providing for remote computer control of the instruments.
In addition to providing a menu-driven interface which permits the operator to program the instrument to set parameters for a sequence of different types of tests, either remotely from a central computer or via the front panel of the instrument, using menus to guide selection of the parameters, and therefore minimize operator error, the present invention also provides interlocks which prevent unsafe electrical connections from being made and which shuts down the test equipment not only upon detection of an over-current at the outputs of the instrument, but also upon detection of an overvoltage in the power supply.
Furthermore, even though the instrument does not include internal power supply circuits for tests other than line leakage tests, but merely allows external connection of testing equipment for the other types of tests, switching to the external connection circuitry is controlled through the same operator interface that controls the line leakage tests, including the selection of human equivalent test circuits, and the external connection circuitry is fully integrated into the safety interlock and overload protection circuitry so as to minimize hazards to the operator.